The polyphenylene ethers and processes for their preparation are known in the art and described in numerous publications including Hay, U.S. Pat. Nos. 3,306,874 and 3,306,875. Other procedures are described in Van Dort, U.S. Pat. No. 3,365,422, Bennett and Cooper, U.S. Pat. Nos. 3,639,656, 3,642,699, 3,733,299, 3,838,102, 3,661,848 and 4,092,294 and Olander, U.S. Pat. No. 4,055,553. All of these patents are incorporated herein by reference.
The processes most generally used to produce the polyphenylene ethers involve the self-condensation of a monovalent phenol in the presence of an oxygen-containing gas and a catalyst.
Improved catalyst systems have been developed for the preparation of polyphenylene ethers by the oxidative coupling of 2,6-di-substituted phenolic compounds. Such systems are disclosed in above-noted U.S. Pat. No. 4,092,294. This patent discloses a catalyst which comprises a copper compound; a diamine wherein the two amino nitrogens are separated by at least two and no more than three carbon atoms and the carbon atom to which the amino nitrogens are attached is aliphatic; a tertiary amine and a bromine-containing compound selected from the group consisting of hydrogen bromide, alkali metal bromides, alkaline earth metal bromides, 4-bromophenols and mixtures thereof plus an amount of a secondary monoamine of the formula ##STR1## wherein R and R' are independently selected from the group consisting of lower alkyl of from 1 to 12 carbon atoms, and preferably lower alkyl of from 3 to 6 carbon atoms.
The preferred secondary amine is di-n-butylamine (DBA). A series of runs using different secondary mono-amines are reported in U.S. Pat. No. 4,092,294 together with the reaction time required for the given system. It is shown therein that the use of two secondary amines (di-n-propylamine and diethylamine) having molecular weights lower than DBA resulted in progressively slower reaction rates as the molecular weight became smaller. Thus, the reaction rate for diethylamine was practically twice as long as the reaction time for DBA. The general teaching apparent from this data is that the lower the molecular weight of the secondary amine, the slower the reaction rate, and, therefore, the less desirable the secondary amine for the practice of the Bennett et al. invention. One of the significant advantages of the Bennett et al. invention is the effective use the practice thereof makes of the DBA, because almost all of the DBA becomes incorporated into the polyphenylene oxide (PPO). Loss of the DBA from the catalyst complex in this manner results in the generation of reactive species that can couple with each other or with other materials (e.g. rubber, high impact polystyrene, etc.) to increase the molecular weight and generate grafted products with such other materials.
Recognizably, it would be desirable to find a low cost replacement for DBA (without any significant loss in the benefits achieved) since the price of DBA and the quantity required make DBA one of the major contributors to catalyst cost. Just such an objective is unexpectedly achieved by the use of a particular secondary mono-amine in accordance with this invention.